The present invention relates to an active filter circuit disposed on the input side of a power supply for improving power factor, and a power supply apparatus with a high power factor including such an active filter circuit.
Most of switching power supplies to which commercial electricity is applied, such as inverters, converters, choppers, etc. include rectifying circuits of a capacitor input-type. A typical example of such rectifying circuits is shown in FIG. 3.
AC current supplied from an AC power supply A is rectified by a diode bridge B, and smoothed by a capacitor C. The wave forms of voltage and current in each part of the circuit in FIG. 3 are shown in FIGS. 4 (a)-(c). When input voltage V.sub.in having a wave form shown in FIG. 4 (a) is supplied from the AC power supply A, the output voltage V.sub.out of the capacitor C has a wave form shown by the solid line in FIG. 4 (b). Here, the dotted line in FIG. 4 (b) shows a wave form of voltage applied to a load Lo when there is no capacitor C. As shown in FIG. 4 (c), input current I.sub.in in flows only when the solid line and the dotted line are superimposed in FIG. 4 (b).
However, in such a circuit, since the input current flows only when the voltage is near its peak as shown in FIGS. 4 (b) and (c), the peak value of the current is inevitably high. If the current including a lot of such higher harmonics flows through a line, the voltage wave form of the AC line is distorted, and the higher harmonics enter into other equipment, resulting in the malfunction of electric devices, etc. and a decrease in the power factor of the AC power supply A.
Various proposals have been made hitherto to solve such problems. There is a system comprising a resistor disposed in an input line. However, this system is disadvantageous in efficiency, improvement of a wave form, variation of rectified voltage due to load current, etc. There is also a simple system comprising an inductor in an input line, which is suitable for small-output power supply. Nevertheless, this inductor-including system is disadvantageous because of a large size of the inductor used, insufficient improvement of a wave form, large variation of rectified voltage due to load current, etc. There is further a system comprising an active filter circuit in an input line, which is most effective and widely used for suppressing higher harmonics. However, this active filter circuit is generally disadvantageous because of an increase in the number of parts used and a high cost.
There is for instance an active filter circuit comprising a transistor constituting a by-pass for current which flows into a capacitor. The rapid switching of the transistor makes an average input current have a nearly sinusoidal wave form. FIG. 5 shows one example of a boosting chopper-type active filter circuit.
This active filter circuit comprises an AC power supply 51, a diode bridge 52, an inductor 53, a diode 54, a resistor 55, a transistor 56 connected between the inductor 53 and the resistor 55, and a capacitor 57 connected between the diode 54 and the resistor 55. Connected in parallel with the capacitor 57 is a load 60. The active filter circuit further comprises a controller 61 connected to the output terminal of the diode 52, the resistor 55, a gate of the transistor 56 and the output terminal of the diode 54. In such an active filter circuit, the high-frequency switching by the transistor 56 makes it possible to reduce the levels of an inductance L and an capacitance C necessary for the active filter. Thus, the choke coil used as inductance L in this circuit can be miniaturized as the switching of the transistor becomes faster.
Used as a choke coil for the active filter circuit may be a ferrite magnetic core with a gap, a silicon-steel magnetic core with a gap, a magnetic core made of an amorphous Fe-base alloy with a gap, a magnetic core made of an amorphous Fe-base alloy or an Fe--Al--Si alloy powder without a gap, etc. However, the ferrite magnetic core of the choke coil for an active filter circuit is likely to be easily saturated, and to exhibit sufficient effects, it is necessary to expand the gap, to increase the number of turns of a coil or to enlarge the size of the magnetic core. The increase in the number of turns of a coil leads to an increase in copper loss and heat generation of the coil, resulting in drastic temperature elevation. On the other hand, if the choke coil is enlarged, the overall circuit cannot be miniaturized.
Since the choke coil using a silicon-steel magnetic core suffers from a large core loss at a high frequency, the high-frequency switching for miniaturizing a choke coil and other parts results in a drastic temperature elevation, meaning that a switching frequency cannot be increased so much. In the case of the choke coil using a magnetic core of an amorphous Fe-base alloy with a gap, the magnetic core should be integrally molded with a resin because the magnetic core is cut to provide a gap. However, since the amorphous Fe-base alloy has a large magnetostriction, the magnetic core suffers from an extremely increased core loss, and noises due to magnetostriction vibration may be generated. In the case of using the magnetic core made of an amorphous Fe-base alloy without a gap, resonance due to magnetostriction vibration may take place, making the operation of the choke coil unstable depending on frequencies, and also noises due to magnetostriction vibration may be generated.
Though the Fe dust core is inexpensive, it has a permeability as low as less than 100, suffering from a large core loss. In addition, a choke coil comprising the Fe dust core cannot be miniaturized so much. A choke coil using the dust magnetic core of an Fe--Al--Si alloy is superior in performance to the choke coil using the Fe dust core, but the dust magnetic core of an Fe--Al--Si alloy cannot be sufficiently miniaturized.
As described above, choke coils used in conventional active filter circuits have their own problems, and it has been desired to provide a high-performance choke coil which is suitable for a high-reliability, high-power factor active filter circuit, and a power supply apparatus with a high power factor including such an active filter circuit.